In the last few years, intensive studies have been conducted on the question of altered immune responses and viruses in Multiple Sclerosis (MS), Subacute Sclerosing Panencephalitis (SSPE), and Progressive Rubella Panencephalitis (PRP). Our studies have focused on specific immune responses to viruses in MS patients and carefully matched controls. We have found no impaired response against measles using three different tests. These tests included direct migration inhibition, complement mediated cytotoxicity and lymphocyte mediated cytotoxicity. A patient with progressive rubella panencephalitis (PRP) was studied and found to have both humoral and cellular (lymphocyte cytotoxicity and lymphocyte stimulation) immunity to measles virus antigens and have normal general immunocompetence. No virus was rescued from this patient.